A mobile communication system for assigning transmission using a scheduler as an HSDPA system standardized in a 3GPP has been partially put to practical use.
Described below is an example of an HSDPA system for performing a high-speed downlink transmission using an example of the configuration of a terminal and an example of a configuration of a base station.
FIGS. 1 through 5 are explanatory views of a conventional HSDPA system.
In the terminal illustrated in FIG. 1, for example, a wireless channel quality measurement/calculation unit 13 measures and calculates a wireless channel quality indicator (hereinafter referred to as a CQI (channel quality indicator)) according to the pilot signal of a downlink signal received through an antenna 10, a radio unit 11, and a demodulation/decoding unit 12. As a practical example, an SIR is calculated by measuring the reception power and interference power of the pilot signal. The CQI value is assembled into a transmitting signal by a wireless channel quality indicator transmission unit 14, encoded and modulated by a coding/modulation unit 15, and transmitted to a base station on an uplink wireless channel through the antenna 10.
On the other hand, the base station illustrated in FIG. 2 receives a signal carrying the CQI value transmitted from a terminal through an antenna 20, a radio unit 21, and a demodulation/decoding unit 22, collects a wireless channel quality indicator (CQI), and notifies a scheduler 24 of the indicator. The scheduler 24 calculates the priority of the terminal for each available frequency band using the wireless channel quality indicator (hereinafter referred to as a CQI (channel quality indicator) reported from the terminal, and selects a transmission parameter on a higher priority basis. A control signal generation unit 25 generates a transmitting control signal, and transmits the signal to a terminal through a coding/modulation unit 27, a radio unit 28, and the antenna 20. The transmission data of a transmission data buffer 26 is transmitted to a terminal after the control signal is transmitted.
FIG. 3 is a flowchart of a scheduling process.
Assume that there are terminals UE1 through UEn in the cell of a base station. In step S10, the CQI values (CQI1 through CQIn) of the terminals UE1 through UEn are received. In step S11, the CQI1 through CQIn are stored. In step S12, a TTI is initialized. A TTI is short for a transmission time interval, and refers to a transmission interval of the data to a terminal. In this example, it is used as a variable indicating a transmission frequency. In step S13, the TTI is increased by 1. In step S14, the priority Pk of the terminal UEk is calculated. In step S15, the system is initialized to i=0, j=1. In step S16, the wireless resources Ri is calculated. With i=0, the wireless resources have not been assigned. Therefore, Ri refers to the entire wireless resources. In step S17, if is determined whether or not the wireless resources Ri is smaller than 0. If the determination in step S17 is YES, control is passed to step S21. If the determination in step S17 is NO, the terminal UEj having the priority Pk of the maximum value Pk_max is calculated from the n−i terminals in step S18. In step S19, the method of transmitting data (data length, modulation system, etc.) to the terminal UEj is selected. In step S20, i is increased by 1, j is increased by 1, and control is returned to step S16. In step S21, the transmitting method selected in step S19 is modulated as a control signal, and the result is transmitted to the terminal. In step S22, the transmission data is modulated for the terminal to which the control signal has been transmitted, transmits the result to the terminal, and control is returned to step S13.
As a method of calculating the priority, the MAX CIR method of selecting a larger CQI value, and the PF (proportional fairness) method of selecting a larger CQI and performing a selection for equal opportunity.
In the above-mentioned 3GPP, the specification of the E3G (evolved 3G) system is inspected as a next generation mobile communication system. In this respect, the implementation of the OFDMA system and the SC-FDMA system are studied respectively for downstream and upstream as a multi-connection method.
In addition, in the E3G system, a scheduling process is performed as with the HSDPA system using the frequency band broader than the conventional HSDPA (for example, four times). Furthermore, the terminal used in the E3G system has different bandwidths between upstream and downstream. Additionally, in the downstream, the available bands by terminals depend on each terminal, for example, 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz, 20 MHz, etc.
Therefore, it is necessary to perform a scheduling process at the system band of 20 MHz by considering the available bandwidth.
That is, as illustrated in FIG. 4, it is necessary to perform the scheduling process on the entire system using one scheduler.
Furthermore, assume that the downlink system bandwidth is 20 MHz, and the downlink bandwidth of a terminal is 5 MHz. At this time, the frequency used during operation is variable with the relationship with other terminals taken into account, and there are four options. Therefore, to allow the scheduler of a base station to select the optimum band from among a plurality of bands with the available bandwidths by other terminals taken into account, the CQI is measured and calculated for every 5 MHz band at a terminal as illustrated in FIG. 5, and the result is to be reported to the base station.
That is, four times as much as the measurement and calculation are required as compared with the HSDPA system. In addition, the frequency of reporting the CQIs to the base stations quadruples. As a result, the interference of the up-channel also quadruples.
In the E3G system, when the entire system is scheduled by one scheduler,
When simply compared with the scheduler of the conventional HSDPA system, the number of terminals to be scheduled is multiplied (for example, quadrupled).
As compared with the transmission interval of 2 msec of the conventional HSDPA system, the interval is ¼, that is, 0.5 msec.
For the two above-mentioned reasons, for example, 16 times scheduling speed as fast as the conventional system is demanded. That is, the priority calculation time is to be set to 1/16.
On the other hand, the improvement of the performance of the process of the CPU and the DSP for performing the scheduling process approximately quadruples on the basis of the reference of year 2010 as the target of starting the service of the E3G, which is far from the above-mentioned 16 times with the Moore's Law (double process speed in 18 months) taken into account.
Therefore, it is inevitable that the scheduling process is performed at a higher speed.
The patent document 1 discloses the technology of grouping and scheduling terminals moving at a high speed. Furthermore, it specifies the bands to be scheduled. It is assumed that they are based on the HSUPA (high speed uplink packet access) of the 3GPP. However, in the descriptions, a terminal moving at a low speed or during halts is not scheduled.
The patent document 2 discloses an example using an OFCDM (orthogonal frequency and code division multiplexing). That is, a spreading process is performed in the frequency and time directions, and then a multiplexing operation is performed.
The patent document 3 groups the terminals using the amount of attenuation of transmission power. Since there are no descriptions about available frequency bands, it is considered that the conventional OFDM is used.
The patent document 4 discloses a base station detecting the moving speed of a mobile station using a Doppler frequency, and optimally selecting a coding rate and a modulation system.
The patent document 5 discloses optimally determining the transmission rate of the communications of a mobile station and a base station according to the information about the Doppler frequency etc. of a mobile station.
The patent document 6 discloses grouping a subcarrier, acquiring channel quality information for each group, and transmitting and receiving the information.
Patent Document 1: Japanese Laid-open Patent Publication No. 2006-060814
Patent Document 2: Japanese Laid-open Patent Publication No. 2005-318434
Patent Document 3: Japanese Laid-open Patent Publication No. 2001-036950
Patent Document 4: Japanese Laid-open Patent Publication No. 2003-259437
Patent Document 5: Japanese Laid-open Patent Publication No. 2005-260992
Patent Document 6: Japanese Laid-open Patent Publication No. 2005-160079